1. Field of the Invention
The invention generally relates to a television system in which digitized television signals are transmitted from an encoding station to a decoding station. More particularly, the invention relates to a television system in which the encoding station is provided with an encoding circuit adapted to perform a transform coding in which groups of picture signal samples are converted into respective groups of coefficients which are transferred to the decoding station. To recover the original television signal samples, this decoding station includes a decoding circuit which is adapted to subject each group of coefficients to an inverse transform.
A system of this type may form part of a television broadcasting system. In that case, the encoding station is incorporated in the television broadcasting transmitter and each TV receiver is provided with a decoding station. In such a case a TV channel is used for transmitting the digitized television signals.
Alternatively, such a system may form part of a video recorder in which case a video tape is used for transmitting the digitized television signals from the encoding station to the decoding station.
2. Description of the Prior Art
As is generally known, a number of fundamental methods of encoding a digital television signal are available to those skilled in the art, such as:
(a) Predictive coding, abbreviated PC; PA1 (b) Transform coding, abbreviated TC.
For performing each of these methods, the television signal is first sampled at a frequency which is usually twice as high as the highest frequency in the signal. This sampling frequency is equal to approximately 10 MHz for a television signal having a bandwidth of approximately 5 MHz. If each sample thus obtained is converted by means of pulse code modulation into an 8-bit PCM word, this yields a bit rate of approximately 80 M bit/second. In practice this bit rate is found to be unacceptably high. In fact, this bit rate requires a transmission channel having a bnadwidth of approximately 40 MHz, which bandwidth is not present in a TV broadcasting channel, let along on a video tape.
A considerably reduction of this bit rate can be obtained by subjecting the PCM words to a predictive coding. As is generally known, see for example reference 1, pages 378-390, a prediction word is subtracted from each PCM word and the difference thus obtained is again subjected to a pulse code modulation. Since 4-bit code words are usually adequate for the representation of these differences, the bit rate is reduced by 50%.
As an alternative to this predictive coding, the PCM words may be subjected to a transform coding, see for example Reference 1, pages 390-396. As is generally known, the television picture is then split up into sub-pictures of NxN pixels. Each sub-picture is subsequently considered as a sum of a plurality of mutually orthogonal basic pictures B(i,k); i, k =1, 2, ... N, each with its own weighting factor y(i, k). As is common practice, these weighting factors will be referred to as coefficients. They are transmitted to the decoding station.
In order to transmit these coefficients to the decoding station with the lowest possible bit rate, they are first subjected to an adaptive coding (see for example reference 2). For coding the coefficients, many bits are assigned to the most significant coefficients, fewer bits are assigned to lesser significant coefficients, while no bits at all are assigned to the least significant coefficients; in other words, these least significant coefficients are not transmitted to the decoding station.
In the case of monochrome TV in which the television signal represents only one quantity varying with respect to time, namely the brightness, the basic picture B(1,1) represents the average brightness of the sub-picture and y(1,1) represents its amplitude. This coefficient y(1,1) is usually the most significant and must therefore be encoded with the greatest accuracy. Eight or nine bits appear to be adequate in practice for this purpose. The other coefficients can usually be encoded with not more than five bits.
By suitable choice of the transform, a bit rate can be realized which is still lower than the bit rate which is obtained by means of predictive coding. The transforms most commonly used in this connection are the Hotelling, the Fourier, the Haar, and the discrete cosine transforms.
A sub-picture can be composed in such a way that it comprises pixels of both the even and the odd field of a TV picture. The transform of such a subpicture is sometimes referred to as intraframe transform. It is alternatively possible to compose a subpicture from pixels all of which are either associated with the even or with the odd field of a TV picture. The transform of such a sub-picture is sometimes referred to as intrafield transform.
Although the general impression is that a bit rate reduction which is by all means interesting can be realized by means of an intraframe transform with only a slight loss of picture quality, practice proves that this reduction is only significant if the pictures to be transformed are still pictures. For moving pictures, an intrafield transform is found to be more efficient than an intraframe transform.